Transflective liquid crystal display with gamma harmonization

ABSTRACT

In a transflective liquid crystal display having a transmission area and the reflection area, the transmissive electrode is connected to a switching element to control the liquid crystal layer in the transmission area, and the reflective electrode is connected to the switching element via a separate capacitor to control the liquid crystal layer in the reflection area. The separate capacitor is used to shift the reflectance in the reflection area toward a higher voltage end in order to avoid the reflectance inversion problem. In addition, an adjustment capacitor is connected between the reflective electrode and a different common line. The adjustment capacitor is used to reduce or eliminate the discrepancy between the gamma curve associated with the transmittance and the gamma curve associated with the reflectance.

FIELD OF THE INVENTION

The present invention relates generally to a liquid crystal displaypanel and, more particularly, to a transflective-type liquid crystaldisplay panel.

BACKGROUND OF THE INVENTION

Due to the characteristics of thin profile and low power consumption,liquid crystal displays (LCDs) are widely used in electronic products,such as portable personal computers, digital cameras, projectors, andthe like. Generally, LCD panels are classified into transmissive,reflective, and transflective types. A transmissive LCD panel uses aback-light module as its light source. A reflective LCD panel usesambient light as its light source. A transflective LCD panel makes useof both the back-light source and ambient light.

As known in the art, a color LCD panel 1 has a two-dimensional array ofpixels 10, as shown in FIG. 1. Each of the pixels comprises a pluralityof sub-pixels, usually in three primary colors of red (R), green (G) andblue (B). These RGB color components can be achieved by using respectivecolor filters. FIG. 2 illustrates a plan view of the pixel structure ina conventional transflective liquid crystal panel, and FIGS. 3 a and 3 bare cross sectional views of the pixel structure. As shown in FIG. 2, apixel can be divided into three sub-pixels 12R, 12G and 12B, and eachsub-pixel can be divided into a transmission area (TA) and a reflectionarea (RA). In the transmission area as shown in FIG. 3 a, light from aback-light source enters the pixel area through a lower substrate 30 andgoes through a liquid crystal layer, a color filter R and the uppersubstrate 20. In the reflection area, light from above an uppersubstrate 20 encountering the reflection area goes through the uppersubstrate 20, the color filter R and the liquid crystal layer before itis reflected by a reflective layer or electrode 52. Alternatively, anon-color filter (NCF) is formed on the upper substrate 20,corresponding to part of the reflective area, as shown in FIG. 3 b.

As known in the art, there are many more layers in each pixel forcontrolling the optical behavior of the liquid crystal layer. Theselayers may include a device layer 50 and one or two electrode layers.For example, a transmissive electrode 54 on the device layer 50,together with a common electrode 22 on the color filter, is used tocontrol the optical behavior of the liquid crystal layer in thetransmission area. Likewise, the optical behavior of the liquid crystallayer in the reflection area is controlled by the reflective electrode52 and the common electrode 22. The common electrode 22 is connected toa common line. The device layer is typically disposed on the lowersubstrate and comprises gate lines 31, 32, data lines 21-24 (FIG. 2),transistors, and passivation layers (not shown). Furthermore, a storagecapacitor is commonly disposed in the device layer 50 to retain theelectrical charge in the sub-pixel after a signal pulse in the gate linehas passed. An equivalent circuit of a typical sub-pixel (m, n) having atransmission area and a reflection area is shown in FIG. 4. In FIG. 4,C_(LC1) is the capacitance mainly attributable to the liquid crystallayer between the transmissive electrode 54 and the common electrode 22,and C_(LC2) is the capacitance mainly attributable to the liquid crystallayer between the reflective electrode 52 and the common electrode 22.C₁ is the storage capacitor and COM denotes the common line.

As it is known in the art, an LCD panel also has quarter-wave plates andpolarizers.

In a single-gap transflective LCD, one of the major disadvantages isthat the transmissivity of the transmission area (transmittance, the V-Tcurve) and the reflectivity in the reflection area (reflectance, the V-Rcurve) do not reach their peak values in the same voltage range. Asshown in FIG. 5, the V-R curve is peaked at about 2.8V, while the “flat”section of the V-T curve is between 3.7V and 5V. The reflectanceexperiences an inversion while the transmittance is approaching itshigher values.

In prior art, this reflectivity inversion problem has been corrected byusing a double-gap design wherein the gap at the reflection area isabout half of the gap at the transmission area. While the double-gapdesign is effective in principle, it is difficult to achieve in practicemainly due to the complexity in the fabrication process. Other attempts,such as manipulating the voltage levels in the transmission and thereflection areas and coating the reflective electrode by a dielectriclayer, have been proposed. For example, the voltage level in thereflection area relative to that in the transmission area is reduced byusing capacitors. As shown in FIG. 6, a separate capacitor C_(C) isconnected in series to C_(LC2). As such, the voltage level on thereflective electrode in reference to the common line voltage levelV_(COM1) is given by: $\begin{matrix}{V_{{CLC}\quad 2} = {{Vcc} - {{Vcom}\quad 1}}} \\{= {\frac{Cc}{( {C_{{LC}\quad 2} + {Cc}} )}*( {V_{data} - {{Vcom}\quad 1}} )}}\end{matrix}$where V_(data) is the voltage level on the data line.

By adjusting the ratio C_(C)/(C_(CL2)+C_(C)), it is possible to shiftthe peak of the reflectance curve toward the higher voltage end so as tomatch the flatter region of the transmittance curve, as shown in FIG. 7a. As such, the inversion in the reflectance relative to thetransmittance can be avoided.

However, while the transmittance starts to increase rapidly at about2.2V, the reflectance remains low until about 2.8V. In this lowbrightness region, the discrepancy in the transmittance and reflectancealso causes the discrepancy between the gamma curve associated with thetransmittance and the gamma curve associated with the reflectance, asshown in FIG. 7 b. FIG. 7 b shows the transmittance and reflectance as afunction of gamma level. Such discrepancy in the gamma curves degradesthe view quality of a transflective LCD panel.

It is thus advantageous and desirable to provide a method to reduce thediscrepancy between the gamma curve associated with the transmittanceand the gamma curve associated with the reflectance.

SUMMARY OF THE INVENTION

The present invention provides a method and a pixel structure to improvethe viewing quality of a transflective-type liquid crystal display. Thepixel structure of a pixel in the liquid crystal display comprises aplurality of sub-pixel segments, each of which comprises a transmissionarea and a reflection area. In the sub-pixel segment, a data line, agate line, a common line connected to a common electrode, and aswitching element operatively connected to the data line and the gateline are used to control the operational voltage on the liquid crystallayer areas associated with the sub-segment. The transmission area has atransmissive electrode and the reflection area has a reflectiveelectrode. The transmissive electrode is connected to the switchingelement to control the liquid crystal layer in the transmission area.The reflective electrode is connected to the switching element via aseparate capacitor to control the liquid crystal layer in the reflectionarea. The separate capacitor is used to shift the reflectance in thereflection area toward a higher voltage end in order to avoid thereflectance inversion problem. In addition, an adjustment capacitor isconnected between the reflective electrode and a different common line.The adjustment capacitor is used to reduce or eliminate the discrepancybetween the gamma curve associated with the transmittance and the gammacurve associated with the reflectance.

The present invention will become apparent upon reading the descriptiontaken in conjunction of FIGS. 8 to 16.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation showing a typical LCD display.

FIG. 2 is a plan view showing the pixel structure of a conventionaltransflective color LCD display.

FIG. 3 a is a cross sectional view showing the reflection andtransmission of light beams in the pixel as shown in FIG. 2.

FIG. 3 b is a cross sectional view showing the reflection andtransmission of light beams in another prior art transflective display.

FIG. 4 is an equivalent circuit of a sub-pixel segment in atransflective LCD panel.

FIG. 5 is a plot of transmittance (T) and reflectance (R) againstapplied voltage (V) in a prior art single-gap transflective LCD.

FIG. 6 is an equivalent circuit of a sub-segment segment in atransflective LCD wherein a separate capacitor is connected to thereflective electrode to reduce the voltage level thereon.

FIG. 7 a is a plot of transmittance (T) and reflectance (R) againstapplied voltage (V) showing the shifting of the R-V curve as a result ofthe separate capacitor in the reflection area.

FIG. 7 b is a plot of transmittance and reflectance as a function ofgamma level.

FIG. 8 is an equivalent circuit of a sub-pixel segment, according to thepresent invention.

FIG. 9 is a timing chart showing the signals at two common lines inrelationship to the gateline signal and the data line signal.

FIG. 10 a is a plot of transmittance and reflectance against appliedvoltage in a sub-pixel segment, according to the present invention.

FIG. 10 b is a plot of transmittance and reflectance as a function ofgamma level, according to the present invention.

FIG. 11 is an equivalent circuit of the transflective LCD displayshowing the driving scheme of COM2, according to the present invention.

FIG. 12 is an equivalent circuit of the sub-pixel segment, according toanother embodiment of the present invention.

FIG. 13 is a timing chart showing the signal at COM2, according to adifferent embodiment of the present invention.

FIG. 14 is a timing chart showing the signals at COM1 and COM2,according to another embodiment of the present invention.

FIG. 15 is a timing chart showing the signals at COM1 and COM2,according to yet another embodiment of the present invention.

FIG. 16 is a cross sectional view showing the layer structure in thelower substrate in a transflective LCD sub-pixel segment, according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

A sub-pixel segment, according to one embodiment of the presentinvention, is illustrated in the equivalent circuit of FIG. 8. As with asub-pixel segment in a prior art transflective LCD display, thesub-pixel segment (m, n), according to the present invention, has atransmission area and a reflection area jointly controlled by the n^(th)gate line and the m^(th) data line via a switching element. Thesub-pixel segment has a common electrode connected to a common lineCOM1. The optical behavior of the liquid crystal layer in the reflectionarea is controlled by the reflective electrode and the common electrode.A storage capacitor C₁ is used to retain the electrical charge in thesub-pixel segment after a signal pulse in the gate line has passed.

In FIG. 8, C_(LC1) is the capacitance mainly attributable to the liquidcrystal layer between the transmissive electrode and the commonelectrode, and C_(LC2) is the capacitance mainly attributable to theliquid crystal layer between the reflective electrode and the commonelectrode. In addition, a separate capacitor C_(C) is connected inseries to C_(LC2) in order to shift the reflectance in the reflectionarea toward a higher voltage end in order to avoid the reflectanceinversion problem. Furthermore, an adjustment capacitor C₂ is connectedbetween the reflective electrode and a different common line nth COM2.The adjustment capacitor is used to reduce or eliminate the discrepancybetween the gamma curve associated with the transmittance and the gammacurve associated with the reflectance. With such an adjustment capacitorC₂, the voltage level on the reflective electrode in reference to thecommon line voltage V_(COM1) is given by: $\begin{matrix}{V_{{CLC}\quad 2} = {{Vcc} - {{Vcom}\quad 1}}} \\{= \frac{{{Cc}*( {V_{data} - {{Vcom}\quad 1}} )} + {( {{Cc} + C_{2}} )*( {{{nth\_ Vcom}\quad 2} - {{Vcom}\quad 1}} )}}{( {C_{{LC}\quad 2} + {Cc} + C_{2}} )}}\end{matrix}$In FIG. 8, COM3 can be the same as COM1 or different from COM1.

The nth V_(COM2) signal on the common line COM2 is shown in FIG. 9. InFIG. 9, the dashed line denotes a reference voltage level V_(REF). Asshown, both the V_(COM1) signal on the common line COM1 and the V_(COM2)source signal are AC signals. While the V_(COM1) signal is substantially180° out of phase with the data signals on Data line n, the V_(COM2)source signal is substantially in phase with the Data line n. It shouldbe noted that the common line COM2 is a floating electrode and,therefore, the shape of nth V_(COM2) signal is dependent upon V_(COM1)and upon the driving mode. For example, when the driving mode is inaccordance with a line inversion scheme, the nth V_(COM2) signal has astep-like shape as shown in FIG. 9. In a negative frame, the nthV_(COM2) signal is, in general, is negative but its amplitudefluctuation follows the shape of V_(COM1). When nth gate line is turnedon again and the frame is positive, the n^(th) V_(COM2) is refreshed andchanges polarity from negative to positive in a pixel. The shape of thenth V_(COM2) remains the same until the next frame.

As seen in the above equation, it is possible to adjust the values ofC_(C) and C₂ to improve the viewing quality of a transflective LCDpanel. For example, it is possible to select C_(C) and C₂ such thatC _(C)/(C _(C) +C _(LC2) +C ₂)=0.46,andC ₂/(C _(C) +C _(LC2) +C ₂)=0.32.

With ΔA_COM=3V (ΔA_COM being the absolute value of the amplitudedifference between nth V_(COM2) and V_(COM1)), the matching between thetransmittance and reflectance is shown in FIG. 10 a. As can be seen inFIG. 10 a, not only the peak of the reflectance curve reasonably matchesthe flatter segment of the transmittance curve at about 4.0V, the slopeof the transmittance curve and the slope of the reflectance curve from2V to 4V region are reasonably close to each other. Based on a 64-leveltransmittance gamma curve with an index of 2.2, or T=(n/64)^(2.2), areflectance gamma curve is obtained as shown in FIG. 10 b. As can beseen, the discrepancy between the transmittance gamma curve and thereflectance gamma curve is greatly reduced.

The nth V_(COM2) signal as shown in FIG. 9 is used for a swing typedisplay in order to achieve a pixel inversion effect. Such a swing typenth V_(COM2) can be realized by using the driving scheme as shown inFIG. 11. As shown in FIG. 11, the adjustment capacitor C₂ iselectrically connected to a common voltage source COM2 through anotherswitching element for receiving nth V_(COM2). In FIG. 11, V_COM1, V₁₃COM3 and V_COM4 can be the same or different. Conveniently, only oneswitching element outside the display area is used to provide the nthV_(COM2) signal for an entire line n. Furthermore, a common capacitorC_(COM) electrically connected to the switching element for stabilizingthe voltage signal at the second common electrode nth COM2. In FIGS. 8and 11, only a common storage capacitor C₁ is used for both thetransmission area and the reflection area in a sub-pixel segment.However, it is possible to have two storage capacitors C_(ST1) andC_(ST2) in a sub-pixel segment, separately storing the electric chargein the transmission area and the reflection area, as shown in FIG. 12.Moreover, it is possible to use a constant V_(COM2) signal, as shown inFIG. 13, rather than the swing type signal of FIG. 9.

In a different embodiment of the present invention, while the swing typenth V_(COM2) is used, V_(COM1) is a constant voltage, as shown in FIG.14. In yet another embodiment of the present invention, both V_(COM1)and nth V_(COM2) are 180° out of phase with Data line n. Thus, V_(COM1)is in phase with nth V_(COM2), as shown in FIG. 15.

The use of adjustment capacitors to achieve harmonization between thetransmittance gamma and the reflectance gamma can be implemented in anActive Matrix transflective liquid crystal display (AM TRLCD) panelwithout significantly increasing the complexity in the fabricationprocess. As shown in FIG. 16, the adjustment capacitor can be realizedby adding a common line COM2 on the lower substrate. By using a floatingmetal layer Metal_1, both C_(C) and C₂ can be achieved.

Thus, although the invention has been described with respect to one ormore embodiments thereof, it will be understood by those skilled in theart that the foregoing and various other changes, omissions anddeviations in the form and detail thereof may be made without departingfrom the scope of this invention.

1. A method for improving viewing quality of a liquid crystal displaypanel, the display panel comprising a liquid crystal layer having afirst side and an opposing second side, and a common electrode disposedon the first side of the liquid crystal layer, wherein the commonelectrode is operatively applied with a common voltage, and wherein thedisplay panel has a plurality of pixels, and at least some of the pixelshave a first area and a second area, the first area comprising a firstelectrode disposed on the second side of the liquid crystal layer, thesecond area comprising a second electrode disposed on the second side ofthe liquid crystal layer adjacent to the first electrode, wherein thefirst electrode is operatively applied with a first voltage to achieve afirst optical transmissivity through the liquid crystal layer in thefirst area as a function of the first voltage, and the second electrodeis operatively applied with a second voltage to achieve a second opticaltransmissivity through the liquid crystal layer in the second area as afunction of the second voltage, wherein the first optical transmissivityhas a lower transmissivity section and a higher transmissivity section,and the second transmissivity has a lower transmissivity section and ahigher transmissivity section, said method comprising: adjusting thesecond voltage relative to the first voltage for substantially matchingthe higher transmissivity section of the second optical transmissivityto the higher transmissivity section of the first opticaltransmissivity, leaving a discrepancy between the lower transmissivitysection of the second optical transmissivity and the lowertransmissivity section of the first optical transmissivity; andproviding a voltage different from the common voltage to the secondelectrode via a charge storage device so as to reduce the discrepancybetween the lower transmissivity section of the second opticaltransmissivity and the lower transmissivity section of the first opticaltransmissivity.
 2. The method of claim 1, wherein the first electrodecomprises a transmissive electrode and the second electrode comprises areflective electrode.
 3. The method of claim 2, wherein the firstoptical transmissivity is equal to the transmittance of the liquidcrystal layer through the transmissive electrode in the first area andthe second optical transmissivity is equal to the reflectance of theliquid crystal layer reflected from the reflective electrode in thesecond area.
 4. The method of claim 2, wherein the first voltage is adata signal.
 5. The method of claim 4, wherein the second area comprisesa charge storage capacitor having a first end electrically connected tothe data signal and a second end operatively connected to the reflectiveelectrode, and the second voltage is a voltage signal at the second endof the charge capacitor.
 6. A method to improve viewing quality of aliquid crystal display, the liquid crystal display comprising: aplurality of data lines for conveying a data signal; a plurality of gatelines for providing a driving signal; and a plurality of pixels, whereineach pixel has a switching unit to admit the data signal from a dataline responsive to the driving signal from a gate line, and wherein eachpixel has a first liquid crystal capacitor and a second liquid crystalcapacitor, wherein a first end of the first capacitor is coupled to theswitching unit, said method comprising: in said each pixel electricallyconnecting a coupling capacitor between the switching unit and a firstend of the second liquid crystal capacitor; applying a first commonvoltage signal to a second end of the first liquid crystal capacitor anda second end of the second liquid crystal capacitor; and electricallyconnecting an adjustment capacitor between the first end of the secondliquid crystal capacitor and a second common voltage signal.
 7. Themethod of claim 6, further comprising: electrically connecting a storagecapacitor in parallel to the first liquid crystal capacitor.
 8. Themethod of claim 6, further comprising: electrically connecting a storagecapacitor in parallel to the second liquid crystal capacitor.
 9. Themethod of claim 6, further comprising: operatively connecting anadditional switching unit between the adjustment capacitor and a voltagesource so as to allow the voltage source to provide the second commonvoltage signal to the adjustment capacitor via the additional switchingunit responsive to the driving signal from the gate line.
 10. The methodof claim 9, further comprising: electrically connecting a furthercapacitor to additional switching unit for stabilizing a voltage at thesecond common electrode.
 11. A liquid crystal display, comprising: aplurality of data lines for conveying a data signal; a plurality of gatelines for providing a driving signal; and a plurality of pixels, eachpixel comprising: a switching unit, responsive to the driving signalfrom a gate line, for admitting the data signal from a data line; afirst common electrode for providing a first common voltage signal; asecond common electrode for providing a second common voltage signal; apixel electrode, electrically connected to the switching unit, fordriving a liquid crystal layer within the pixel based on the admitteddata signal and the first common voltage signal; a first liquid crystalcapacitor having a first end electrically connected to the first commonelectrode, and a second end electrically connected to the pixelelectrode; an coupling capacitor having a first end and a second end,wherein the first end is electrically connected to the pixel electrode;an adjustment capacitor having a first end electrically connected to thesecond common electrode and a second end connected to the second end ofthe coupling capacitor; and a second liquid crystal capacitor having afirst end electrically connected to the first common electrode, and asecond end electrically connected to the second end of the couplingcapacitor, for driving the liquid crystal layer based on the admitteddata signal, the first common voltage signal and the second commonvoltage signal.
 12. The liquid crystal display of claim 11, furthercomprising a storage capacitor connected in parallel to the first liquidcrystal capacitor in said each pixel.
 13. The liquid crystal display ofclaim 11, further comprising a storage capacitor connected in parallelto the second liquid crystal capacitor in said each pixel.
 14. Theliquid crystal display of claim 11, wherein, in said each pixel, thefirst liquid crystal capacitor has two electrode ends, each of which ismade of substantially transparent material, and the second liquidcrystal capacitor has a first electrode end made of substantiallytransparent material and a second electrode end made of a reflectivematerial.
 15. The liquid crystal display of claim 11, wherein, in saideach pixel, the second liquid crystal capacitor has two electrode ends,each of which is made of substantially transparent material, and thefirst liquid crystal capacitor has a first electrode end made ofsubstantially transparent material and a second electrode end made of areflective material.
 16. The liquid crystal display of claim 11, furthercomprising: a voltage source for providing the second common voltagesignal; and an additional switching unit, responsive to the drivingsignal from said gate line, for electrically connecting the secondcommon electrode in said each pixel to the voltage source.
 17. Theliquid crystal display of claim 16, further comprising a furthercapacitor electrically connected to the second common electrode in saideach pixel for stabilizing a voltage at the second common electrode. 18.The liquid crystal display of claim 17, wherein the further capacitorhas a first end electrically connected to the second common electrodeand a second end electrically connected to the first common electrode.19. The liquid crystal display of claim 16, wherein each of the firstand second common voltage signals is a constant voltage signal or an ACvoltage signal.
 20. The liquid crystal display of claim 16, wherein thefirst common voltage signal and the second common voltage signal are ACsignals 180 degrees out of phase with each other.
 21. The liquid crystaldisplay of claim 16, wherein the first common voltage signal and thesecond common voltage signal are AC signals in phase with each other.22. The liquid crystal display of claim 16, wherein the second commonvoltage signal comprises a constant voltage signal.
 23. The liquidcrystal display of claim 11, wherein the first end of the first liquidcrystal capacitor comprises a first transparent electrode and the secondend of the first liquid crystal capacitor comprises a second transparentelectrode, and wherein the first end of the second liquid crystalcapacitor comprises a transparent electrode and the second end of thesecond liquid crystal capacitor comprises a reflective electrode. 24.The liquid crystal display of claim 23, further comprising: a voltagesource for providing the second common voltage signal; an additionalswitching unit, responsive to the driving signal from said gate line,for electrically connecting the second common electrode in said eachpixel to the voltage source; and a further capacitor electricallyconnected to the second common electrode for stabilizing a voltage atthe second common electrode.